<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>099-布局-固定定位实现蒙版.html</title>
	<style>
		*{
			margin: 0;
			padding: 0;
		}
		.box{
			position: fixed;
			left: 0;
			top: 0;
			bottom: 0;
			right: 0;
			background: rgba(0,0,0,0.3);
		}
		body{
			height: 4000px;
		}
	</style>
</head>
<body>
	<div class="box"></div>
	<a href="#">点我试试</a>
</body>
</html>